1. Field of the Invention
The invention relates to a process for obtaining fatty-acid esters from fat and/or oil of biological origin by transesterification.
Transesterification reactions as such are known; they represent a commercially important class of industrial organic reactions. In the course of a transesterification reaction an ester is converted into another ester by exchange of the acid groups or by exchange of the alcoholic groups. If the transesterification is carried out by exchange of the alcoholic groups, one speaks of so-called alcoholysis (also alkanolysis). In the case of alcoholysis the alcohol is added in excess in order to obtain a high yield of desired ester. In connection with the generation of diesel fuel from renewable raw materials the production of alkyl esters, particularly of methyl esters, by alcoholysis of vegetable oils (e.g. rapeseed oil) has recently gained considerably in topicality.
2. Description of the Related Art
Transesterification is an equilibrium reaction which, as a rule, is already triggered by mixing the reactants. However, the reaction proceeds so slowly that a catalyst is required for commercial implementation of the reaction. Conventionally use is made of strong acids or strong bases by way of catalysts.
Fats and oils of biological origin consist predominantly of glycerides (monoglycerides, diglycerides and triglycerides). In the course of the transesterification of such fats and oils the component glycerin can be substituted by low-molecular monohydric alcohols. In practice the method according to Bradshaw (described in U.S. Pat. Nos. 2,271,619 and 2,360,844) is frequently adopted for this purpose. The reaction is carried out in an open container which may consist of ordinary carbon steel. The fat or oil has to be dry (anhydrous), clean and, above all, neutral, i.e. the content of free fatty acids must be negligibly small (acid value no higher than 1.5). The initial product is heated to about 80xc2x0 C., then commercial anhydrous methanol (99.7 wt. %), which contains 0.1 to 0.5 wt. % sodium hydroxide or potassium hydroxide in dissolved form, is added in excess (about 1.2 to 1.6 times the stoichiometric quantity). After addition of the alcohol the mixture is stirred for a few minutes and then allowed to stand. The glycerin begins to settle immediately. Since it is practically free from water and much heavier than the other liquids, it settles readily and forms a layer on the bottom of the container. The reaction of an oil with methanol to form methyl ester is usually 98% complete after one hour. The bottom layer (lower layer) contains not less than 90% of the glycerin originally present in the fat. The upper layer is composed of methyl esters, unconverted alcohol and alkali, the remaining glycerin, and a very small amount of soap. These various contaminants are washed out of the esters by repeated washing with small quantities of warm water. The method is advantageous, because methyl esters or ethyl esters can be obtained from the fat directly without any intermediate step, the reaction temperature is low and no items of apparatus made of special corrosion-resistant material are required.
With the method according to Bradshaw the methyl esters that are obtained are subsequently used in a continuous process for the production of anhydrous soap. For this purpose the esters are saponified at low temperature by means of sodium hydroxide and potassium hydroxide and the readily volatile methanol that is released in the process is recovered.
Feuge and Gros studied the transesterification of peanut oil with ethanol (J. Am. Chem. Soc. 26 [1949] 97-102). They found that the optimal temperature for the reaction lies in the vicinity of 50xc2x0 C. A higher yield of glycerin was obtained at this temperature than at 30xc2x0 C. or 70xc2x0 C.
Toyama et. al. (Y. Toyama, T. Tsuchiya and T. Ishikava, J. Soc. Chem. Ind. Japan, 36 [1933] 230-232B) showed that the equilibrium between methanol or ethanol and fats in the presence of sodium hydroxide is attained at room temperature within two hours. In order to take the reaction as far as complete conversion of the fat into monoester, the glycerin that is released has to be removed.
In a paper by Wright et. al. (H. J. Wright, J. B. Segur, H. V. Clark, S. K. Coburn, E. E. Langdon and R. N. DuPuis, Oil and Soap, 21 [1944] 145-148) the exact conditions for the alcoholysis of fats with methanol and ethanol were investigated in detail. Moreover, a report is given by the authors about experiments on alcoholysis with other monohydric alcohols. It is stated that the alcoholysis as described above, which is catalysed with alkali, is only completely successful when the fat is virtually free from free fatty acids and the reaction mixture is free from water. If one of these conditions is not satisfied, saponification occurs, which results in a loss of alkalinity and the formation of a gel structure which prevents or slows down the separation and settling of the glycerin.
Difficulties arise in the course of ethanolysis if the content of free fatty acids in the fat amounts to more than about 0.5 wt. %. If 30 parts of ethanol, 100 parts of cottonseed oil and 0.5 wt. % sodium hydroxide are caused to react, the yield of glycerin is diminished considerably by virtue of 0.3 wt. % water in the reaction mixture. However, the effect of the moisture can be partly compensated by adding further alkali and/or alcohol. If the catalyst content is doubled or the quantity of alcohol is increased to 40 parts, the water content in the reaction mixture may amount to up to 0.6 wt. %.
Similarly, it is shown by Wright et. al. that the rate of the overall reaction is fundamentally limited by the time that the glycerin requires for separation by gravity. A continuous separation by centrifuging at 65xc2x0 C. with a dwell-time of about 5 minutes yielded a rather good result of approximately 85% of the theoretical value. The statement by Bradshaw and Meuly that less alcohol is needed in the case of stepwise addition of the alcohol and removal of the glycerin arising was confirmed for methanolysis but not for ethanolysis, in the case of which this procedure leads to gelling.
Various problems arise in particular when use is made of sodium compounds and potassium compounds by way of catalysts in the course of the transesterification of triglycerides with methanol and ethanol. For instance, by reason of the emulsion that has formed in the reaction, after the transesterification reaction has taken place the two phases separate at such a slow rate that the separation is very prolonged and large reaction volumes are necessary. In addition, even after the phase separation very fine droplets of glycerin still remain suspended in the monoester phase, which have to be washed out with water. Furthermore, the catalyst, which is distributed in both phases, has to be removed at least from the monoester phase after the reaction has been concluded. Depending on the further use of the glycerin, it is furthermore necessary to remove the dissolved catalyst also from the glycerin phase. It is regarded as an additional problem that the reaction sometimes does not set in immediately after the mixture has been produced.
A number of processes have been proposed with a view to solving the stated problems. For instance, in U.S. Pat. No. 2,383,614 a process for the continuous alcoholysis of fat is described in which a partial esterification of the fat or oil is carried out, optionally in several steps, and deposition of the glycerin is also undertaken in several stages. According to U.S. Pat. No. 2,383,580, after completion of the reaction firstly the catalyst that is used is inhibited by neutralisation of the reaction mixture and then the excess alcohol is removed by distillation. The remaining reaction mixture is distilled in a vacuum. In the process the condensate separates into a layer of glycerin and a layer of fatty-acid alkyl ester.
According to the method proposed in U.S. Pat. No. 4,164,506, in a two-stage process firstly the free fatty acids are converted into the corresponding esters with short-chain alcohols in the presence of acidic catalysts. After this, transesterification of the glycerides is undertaken in the presence of alkali, accompanied by deposition of glycerin.
In German published application DE 34 21 217 A1 a process is described for producing fatty-acid esters of short-chain primary and secondary alcohols with 1 to 5 carbon atoms by transesterification of glycerides. In this process a stream of the gaseous alcohol is passed through the liquid glycerides at temperatures between 230 and 270xc2x0 C. With this stream the product mixture consisting of glycerin and fatty-acid alkyl ester is discharged from the reaction zone and subsequently separated. By the action of a catalyst, alkali is dissolved in the liquid glycerides of the reaction vessel.
According to the process described in German patent specification DE 198 03 053 C1, triglycerides are converted with alcohol in multiple molar excess in the presence of suitable catalysts, such as zinc soaps for example, preferably in parallel-flow columns at temperatures from 200 to 240xc2x0 C. and at pressures up to 90 bar. After separation of the excess alcohol, mixtures of alkyl ester and glycerin are obtained which are separated in a separator into the lighter organic phase and into the glycerin phase. This phase separation is directly followed by further processing and cleaning of the products. The ester phase is washed with water in order to remove the glycerin residues dissolved in the product. In this process about 40% of the zinc soaps dissolved in the ester by way of catalyst are also washed out in the form of zinc hydroxide.
Furthermore, investigations have been carried out with the aim of replacing the sodium compounds and potassium compounds with basic ammonium compounds by way of catalyst or reactant. Schuchardt et. al (J. Braz. Chem. Soc. 9 [1998] 199-210) investigated the activity of numerous non-ionic bases with regard to their suitability as catalyst for the alcoholysis of fats and oils, e.g. amines such as triethylamine, piperidine, 1,2,2,6,6,-pentamethylpyridine, pyridine, 2,6-di-tert.-butylpyridine, 4-dimethylaminopyridine (DAMP), guanidines such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 1,1,3,3-tetra-methylguanidine (TMG), 1,1,2,3,3,-pentabutylguanidine (PBG), 1,3-diphenylguanidine, 1,2,3-triphenylguanidine and other aminoguanidines and nitroguanidines, triamino(imino)phosphoranes such as tert.-butylimino-2-diethylamino-1,3-perhydro-1,2,3-diazaphosphorane (BEMP), tris(dimethylamino)methylimino-phosphorane (Me7P). The latter are frequently used in organic synthesis. In one series the catalytic activity of a number of guanidines, e.g. the amidines DBU and DBN, and of the phosphoranes BEMP and Me7P was compared with other bases. The guanidines are the more active catalysts. Their activity conforms to their relative basicity. The activity of TBD at a concentration of 3 mol % was similar to that of 3-mol % potassium carbonate.
The advantage that guanidines offer in the transesterification of fats and oils of biological origin consists in the possibility of heterogenising them on organic polymers and therefore of attaining heterogeneous catalysis. Schuchardt et. al. investigated cellulose, polystyrene/divinylbenzene, and polyurethanes with respect to their suitability as carrier material for guanidines. The heterogenisation of guanidines on organic polymers and their use in the transesterification of vegetable oils are described in Brazilian Patent 8202429 (1984, inventors U. Schuchardt and O. C. Lopes). The guanidines, which were bound to gel-like poly(styrene/divinylbenzene) or cellulose, showed a slightly reduced activity in comparison with the catalytic reaction in homogeneous phase. But they allow the same high conversions after extended reaction-times. Although less active than their homogeneous analogues, all the guanidines containing polymer were able to be re-used in a number of consecutive reaction cycles. However, a loss of activity was observed;,already after about 9 reaction cycles an abatement of the activity could be detected. This decline in activity upon repeated use was mainly caused by the slow lixiviation of the anchored base out of the polymers.
State of the art is, moreover, the catalytic acceleration of the transesterification of methyl phthalates with glycols by zinc acetate and other acidic catalysts. The alcoholysis of fats and oils with the aid of zinc soaps as catalysts at high temperatures (above 210xc2x0 C.) is described in the aforementioned DE 198 03 053 C1. Since heavy-metal soaps, such as zinc acetate for example, are soluble in the reaction mixture consisting of alkanol, oil, fatty acid, alkanol ester and glycerin, heterogeneous catalysis is not possible. In addition to this, at lower temperatures, e.g. at 850xc2x0 C., no noticeable conversions with zinc soaps as catalyst were found.
From the foregoing it follows that the production of simple fatty-acid esters from fat and/or oil of biological origin has not been solved satisfactorily heretofore. Fats and oils of biological origin also always contain, in addition to glycerides, free fatty acids, with which the catalysts that are traditionally used (in particular, alkali oxides or alkali alcoholates) are not compatible or are only poorly compatible. Free fatty acids that are present in the initial product therefore have to be removed, with the exception of residual concentrations of 0.1 wt. % and less, prior to the transesterification, particularly if use is to be made of alkaline catalysts. If the initial product contains a large quantity of free fatty acids, chemical deacidification is too elaborate, and deacidification is then conventionally effected by means of so-called steam stripping at 240 xc2x0 C. to 260xc2x0 C. The high temperature requires a corresponding expenditure of energy.
The object underlying the invention is therefore to provide a heterogeneously catalysed process for the transesterification of fat and/or oil of biological origin, wherein the catalyst that is used is stable and the products arising are not contaminated by catalyst material. In particular, with the new process the free fatty acids that are present in the initial material are also to be converted into esters, in order in this way to eliminate the deacidification that has been necessary hitherto. The new process is simple to carry out and results in good yields.
The present invention involves placing the fat and/or oil of biological origin in a vessel. Alcoholysis is induced by the addition of an alcohol, preferably a monohydric alcohol, and a catalyst to the fat and/or oil that has been provided. The catalyst is a metal salt of an amino acid or of an amino acid derivative. The metal salt is insoluble in alkanols.
Surprisingly, it was accordingly found that the zinc salt of arginine, for example, is insoluble in methanol and, furthermore, in mixtures of glycerides, glycerin, methyl esters and methanol such as are found in the course of methanolysis. After extraction for 4 hours with methanol in a Soxhlet apparatus, no weight loss could be detected. Consequently this compound is well suited as a catalyst for heterogeneously catalysed transesterification. In the course of methanolysis with zinc arginate as catalyst at 85xc2x0 C. and with an addition of 6 mole equivalents of methanol to deacidified palm oil during the course of 180 minutes a yield of 45% methyl esters was obtained. Under the same conditions with zinc glycinate as catalyst a yield of only 0.5% methyl esters was obtained with a contact-time of 180 minutes. The higher activity of zinc arginate at low temperatures might be attributable to the higher basicity of arginine in comparison with glycine. At 135xc2x0 C. the difference in the catalytic activity between zinc arginate and zinc glycinate is less than at 70xc2x0 C. Zinc arginate is distinguished in that it is soluble neither in the methanol nor in the fats and oils; it can consequently serve as a basis for heterogeneously catalysed alcoholysis. Analogous behaviour is shown by the arginate of cadmium.
Zinc arginate can be compressed into relatively stable pellets and installed into a tubular reactor in this form as packing. Another processing mode consists in the pulverulent, finely crystalline metal arginate being suspended in the mixture of oil and alkanol. After passing through a cascade of stirred-tank reactors, the catalyst is filtered off or separated with a centrifuge and recirculated. Subsequently the mixture that has been formed in the course of the reaction, consisting of excess methanol, glycerin and, optionally, water (in the case of an higher concentrations of free fatty acids in the feed), is lava decanted from the methyl-ester phase. The esters that have been formed are freed from unsaponifiable and unconverted residues and purified by distillation.
Moreover, it turned out that heavy-metal salts of amino acids, which are insoluble in the reaction mixtures consisting of glycerides, alcohols, fatty-acid alkyl esters and glycerin, catalyse the alcoholysis of glycerides also in the presence of free fatty acids. Preferred metals are: zinc, lanthanum, cadmium, iron, etc. In this connection the fats and oils arising for the purpose of transesterification may contain larger quantities of free fatty acids. Even at concentrations of free fatty acids amounting to around 10 wt. %, the rate of the transesterification reaction with the catalysts according to this invention at temperatures above 100xc2x0 C. is sufficient for commercial application. Preferred alkanols for the process according to this invention are primary and secondary alkanols with 1 to 6 C atoms.
In the presence of higher concentrations of free fatty acids the rate of reaction is somewhat diminished in comparison with the reaction with fat-free neutral oils. By increasing the alkanol concentration and/or the temperature and/or the catalyst concentration, the decline in the activity of the catalyst can be compensated and a rate of reaction that is sufficient for practical purposes can be obtained.
Further investigations showed that the heavy-metal salts of betaine, carnitine, and taurine, which contain quaternary nitrogen, are also suitable as catalysts for the transesterification of glycerides with alkanols. At temperatures above 100xc2x0 C. they show good catalytic activity. The lanthanum salt of taurine is attractive on account of its stability when subjected to temperature loading and its insolubility in the reaction media entering into consideration.
The catalysts according to this invention do not form with the free fatty acids any soaps that render the separation of the glycerin difficult by virtue of emulsifier action. After a contact-time of 93 hours no abatement of the catalytic activity of zinc arginate could be observed at 135xc2x0 C. No diminution of the concentration of by-products, such as tocopherols, tocotrienols and carotenes, was observed at the relatively high temperature of 135xc2x0 C., even with reaction-times of more than 4 hours.
In the course of the esterification of triglycerides in the batch process in the case of a higher content of free fatty acids it is occasionally advantageous to add the alkanol in stepwise manner. In the first step about ⅓ of the overall quantity of alkanol provided is added and a period of waiting is observed until the alkanol has been converted. After this, the next portion is added and again a short waiting-period is observed. This procedure is continued until the entire amount of alkanol has been caused to react.
In the course of the transesterification according to the invention of triglycerides that contain relatively large quantities of free fatty acids, the water that is formed together with glycerin in the mixture is precipitated out with glycerin as a separate, heavy phase. The mixture of glycerin and water is decanted. Since the formation of soaps is ruled out in the case where use is made of a solid catalyst, the joint formation of glycerin and water does not give rise to any increase in the tendency towards emulsification. The separation of the reaction mixture into two liquid phases takes place quickly and completely at temperatures above 100xc2x0 C. Two co-existing clear liquid phases arise. At lower temperatures the use of a centrifuge is advisable.
Conventional alkanolysis, which is accelerated with basic catalysts such as sodium methylate, sodium hydroxide, potassium methylate, potassium hydroxide etc, requires the free fatty acids to be removed from the initial material as extensively as possible. Since the fats and oils that are derived from oil-bearing seeds only in rare cases contain free fatty acids in a concentration of more than 20 wt. %, in the course of the alkanolysis according to the process of this invention a deacidification of the initial product is generally superfluous. The omission of this intermediate step represents a significant cost-saving in respect of operating resources, investments and effort.
Ifxe2x80x94as proposed in accordance with the inventionxe2x80x94use is made of salts that are insoluble in the reaction mixture, the advantages of a heterogeneous mode of processing can be utilised. In this case the two liquid phases arising in the course of the alcoholysis contain no catalyst, markedly simplifying their processing. Since the catalysts according to this invention are temperature-resistant up to about 200xc2x0 C., it is possible to carry out the alcoholysis in the temperature range up to 180xc2x0 C. without any side-reaction occurring. Finally, the rate of reaction at temperatures above 120xc2x0 C. is so high that continuous process control is possible.
A number of amino acids that are suitable for the process according to this invention are listed below: arginine, asparagine, carnitine, creatine, betaine, dimethylglycine, glycine, lysine, ornithine, taurine etc. The heavy-metal salts of these amino acids that are insoluble in the alkanols are suitable as catalysts for the heterogeneous transesterification of fats and oils with alkanols.